● In nature, we rarely find `color{violet}("isolated, single individuals")` of any species; `color{violet}("majority ")`of them live in groups in a well defined `color{violet}("geographical area, share or compete")` for similar `color{violet}("resources, potentially interbreed")` and thus `color{violet}("constitute a population.")`

● Although the term `color{violet}("interbreeding implies sexual reproduction")`, a group of individuals resulting from even `color{violet}("asexual reproduction")` is also generally considered a `color{violet}("population")` for the purpose of `color{violet}("ecological studies.")`

● All the cormorants in a wetland, rats in an `color{violet}("abandoned dwelling, teakwood trees")` in a forest tract, bacteria in a `color{violet}("culture plate")` and `color{violet}("lotus plants")` in a `color{violet}("pond,")` are some examples of a `color{violet}("population.")`

● Although an `color{violet}("individual organism")` is the one that has to cope with a `color{violet}("changed environment")`, it is at the population level that natural selection operates to evolve the `color{violet}("desired traits")`.

● `color{violet}("Population ecology")` is, therefore, an important area of `color{violet}("ecology")` because it links ecology to `color{violet}("population genetics")` and `color{violet}("evolution.")`

● A `color{violet}("population")` has certain attributes that an individual `color{violet}("organism")` does not.

● An individual may have `color{violet}("births")` and `color{violet}("deaths")`, but a population has `color{violet}("birth rates")` and `color{violet}("death rates.")`

● In a `color{violet}("population")` these rates refer to per `color{violet}("capita births")` and `color{violet}("deaths,")` respectively.

● The rates, hence, are expressed is change in numbers `color{violet}("(increase or decrease)")` with respect to members of the population.

● Here is an example:

● If in a pond there are `color{violet}("20 lotus plants last year")` and through `color{violet}("reproduction 8 new plants")` are added, taking the `color{violet}("current population to 28")`, we calculate the `color{violet}("birth rate as 8/20 = 0.4")` offspring per lotus per year.

● If 4 individuals in a `color{violet}("laboratory population")` of `color{violet}("40 fruitflies died")` during a specified time interval, say a week, the `color{violet}("death rate")` in the `color{violet}("population")` during that period is `color{violet}("4/40 = 0.1")` individuals per fruitfly per week.

● Another `color{violet}("attribute characteristic")` of a population is `color{violet}("sex ratio")`.

● An individual is either a male or a female but a `color{violet}("population")` has a `color{violet}("sex ratio")` (e.g., 60 % of the population are females and 40 % males).


● A `color{violet}("population")` at any given time is composed of individuals of `color{violet}("different ages")`.

● If the `color{violet}("age distribution")` ( % individuals of a given age or age group) is plotted for the population, the resulting structure is called an `color{violet}("age pyramid")`.

● For human population, the `color{violet}("age pyramids")` generally show `color{violet}("age distribution")` of males and females in a `color{violet}("combined diagram")`.

● The `color{violet}("shape ")`of the `color{violet}("pyramids")` reflects the growth status of the population –

(a) `color{violet}(" it is growing")`

(b) `color{violet}("stable")`

(c) `color{violet}("declining.")`

● The size of the population tells us a lot about its status in the habitat.

● Whatever `color{violet}("ecological processes")` we wish to investigate in a `color{violet}("population,")` be it the outcome of competition with another species, the impact of a `color{violet}("predator ")`or the effect of a `color{violet}("pesticide application,")` we always evaluate them in terms of any change in the `color{violet}("population size.")`

● The size, in nature, could be as low as <10 (`color{violet}("Siberian cranes at Bharatpur wetlands in any year")`) or go into millions (`color{violet}("Chlamydomonas in a pond")`).

● Population size, more technically called `color{violet}("population density (designated as N)")`, need not necessarily be measured in numbers only.

● Although total number is generally the most appropriate measure of `color{violet}("population density")`, it is in some cases either `color{violet}("meaningless or difficult")` to determine.

● In an area, if there are `color{violet}("200 Parthenium plants")` but only a single huge banyan tree with a large canopy, stating that the `color{violet}("population density")` of banyan is low relative to that of `color{violet}("Parthenium")` amounts to underestimating the enormous role of the Banyan in that community.



● In such cases, the percent cover or `color{violet}("biomass")` is a more meaningful measure of the `color{violet}("population size.")`

● Total number is again not an easily adoptable measure if the `color{violet}("population")` is huge and counting is impossible or very time-consuming.

● Sometimes, for certain `color{violet}("ecological investigations"0`, there is no need to know the `color{violet}("absolute population densities")`; relative densities serve the purpose equally well.

● For instance, the number of fish caught per trap is good enough measure of its `color{violet}("total population density")` in the lake.

● We are mostly obliged to estimate `color{violet}("population sizes indirectly")`, without actually counting them or seeing them.

● The `color{violet}("tiger census")` in our national parks and tiger reserves is often based on`color{violet}(" pug marks")` and `color{violet}("fecal pellets.")`